The present invention relates to methods for altering residual tensile stresses in a metal surface by using mechanically induced liquid cavitation and particularly relates to methods for either reducing the tensile stresses or converting the tensile stresses to compressive stresses in the metal surface using mechanically induced liquid cavitation.
Conventional welding processes, as well as other surface-affecting processes such as abrasive grinding, standard machining and electrical discharge machining (EDM) typically result in tensile values of surface residual stresses in a metal surface, e.g., a weld deposit and heat affected zones (HAZ). The value of these tensile stresses is often high, approaching or even exceeding the yield strength of the material. These residual stresses have often led to stress-corrosion cracking (SCC) in susceptible materials, particularly those exposed to the coolant in boiling water nuclear reactors. It is highly desirable to treat these welds and other high tensile-stress areas in either old or new applications so as to prevent SCC initiation, which requires the presence of a tensile surface stress.
In addition to SCC, tensile surface residual stresses can increase the risk of fatigue cracking initiation. Conventional mechanical peening with a peening hammer or shot blasting are known methods of changing a tensile surface residual stress (or a near-zero level of stress) to that of high compression. However, this change in stress is accompanied by a significant degree of undesirable surface and subsurface plastic flow (also known as cold-work). Conventional mechanical peening is unacceptable in many materials which are also susceptible to SCC, since significant amounts of surface cold-work are known to make otherwise SCC-resistant microstructures become susceptible to crack initiation when subjected to aggressive environments.
Several methods are known for improving the surface residual stress in metallic components. A prior method for decreasing tensile surface residual stress utilizes ultra-high pressure water jet peening to produce cavitation near the underwater treatment area. This area has the significant disadvantage of being restricted to partially or fully submerged components. This problem results from the need to allow unrestricted flow of the high-pressure water to the submerged (or internally flooded) work surface. This known method to change these residual tensile surface stresses to compressive stresses with the use of ultra high pressure water-jet peening is a locally-applied, mobile process, and has been developed for submerged component use only.
The water-jet stress modification method requires extremely expensive, massive water pumping and piping equipment, and produces a substantial reaction force on the jet nozzle. In addition, the degree of control of the rarefaction and compression periods of the pressure waves which create cavitation is relatively poor, since the wave driving forces (jet pressure and velocity at the nozzle) are predetermined constants, and the formation of pressure waves and corresponding cavitation is merely a by-product of the turbulence resulting from dissipation of the water jet velocity, and not a directly programmed parameter. Water-jet peening therefore is practically limited to more critical applications where the negative factors of high equipment cost, complex delivery system, minimal cavitation control, and low efficiency are sufficiently justified. It is also limited to those areas where sufficient equipment access exists for the required nozzle size and stand-off distance from the work surface.
Another method known for improving surface residual stresses is peening with repeated impact of a hammer or high-velocity shot; however, this method is prone to cause excessive plastic deformation (also known as “cold working”) of the surface and near-surface material due to the severity of the impact mechanical forces. Use of a peening hammer has the disadvantage of not conforming closely to an uneven work surface, and therefore not providing uniform compression over the treated area, even with multiple hits. Use of shot blasting has the disadvantage that the used shot can readily become a contaminant in the area where the process is applied, even with a shot scavenging system.
A third method known for improving surface residual stress utilizes a pulsed laser beam directed at a submerged work surface, which is very locally and rapidly heated by the focused beam. The “Q-Switched” laser pulse forms a vapor cavity that is restrained from free expansion by the surrounding liquid, and therefore rapidly collapses at the work surface to generate a fluid compressive wave which in turn generates a permanent compression of the work surface after the wave dissipates. The vapor bubble is formed as the laser power sublimates a portion of the substrate surface material and working fluid or, preferably, a process coating applied to this surface. This tedious method also requires a submerged component, and must typically have an optically-absorptive surface coating (such as black paint) for effective laser heating. It is appropriately applied to smaller components which can be readily coated in a dry environment, submerged in liquid, and then laser-stress improvement treated. Contamination of the work surface by the optical coating may also be a problem either during the stress improvement treatment, or later when the component is put in service in a contamination-controlled environment. Accordingly, there is a need for altering residual tensile stresses in a metal surface with mechanically-induced liquid cavitation.